Energy Conversion and Management 45 (2004) 1369–1378
Experimental evaluation of a non-azeotropic working
ﬂuid for geothermal heat pump system
L. Zhao *
Department of Thermal Energy and Refrigeration Engineering, School of Mechanical Engineering,
Tianjin University, No. 92 Weijin Road, Tianjin 300072, PR China
Received 27 May 2003; accepted 1 September 2003
Geothermal energy resources are found in many countries. A reasonable and eﬃcient utilization of these
resources has been a worldwide concern. The application of geothermal heat pump systems (GHPS) can
help increase the eﬃciency of using geothermal energy and reduce the thermal pollution to the earth
surface. However, this is only possible with a proper working ﬂuid. In this paper, a non-azeotropic working
ﬂuid (R290/R600a/R123) is presented for a GHPS where geothermal water at 40–45 °C and heating net-
work water at 70–80 °C serve as the low and high temperature heat sources. Experimental results show that
the coeﬃcient of performance (COP) of a GHPS using the working ﬂuid is above 3.5 with the condensation
temperature above 80 °C and the condensation pressure below 18 bar, while the temperature of the geo-
thermal water is reduced from 40–46 °C to 31–36 °C.
Ó 2003 Published by Elsevier Ltd.
Keywords: GHPS; Non-azeotropic working ﬂuid; Experimental study
Geothermal energy resources are found in many countries, but most of them belong to the
category of intermediate or low temperature below 100 °C that is often used in winter heating
[1,2]. The application of heat with geothermal water helps reduce the use of coal and, thus, im-
proves air quality. However, the geothermal water is usually discharged at 40–50 °C directly, since
low enthalpy energy is relatively more diﬃcult to extract. This is frequently observed, especially in
Tel.: +86-22-2740-5049; fax: +86-22-2789-0050.
E-mail address: firstname.lastname@example.org (L. Zhao).
0196-8904/$ - see front matter Ó 2003 Published by Elsevier Ltd.
1370 L. Zhao / Energy Conversion and Management 45 (2004) 1369–1378
developing countries . This practice not only results in low eﬃciency of using geothermal energy
resources but also causes pollution to the earth surface .
In order to solve the problem, a geothermal heat pump system (GHPS) is often used . Among
the many types of heat pump systems, the vapor compression heat pump system is more widely
adopted for its mechanical simplicity and higher eﬃciency. It is also the type that is under study in
The vapor compression GHPSÕ low temperature source is geothermal water at 40–45 °C, and its
high temperature source is clear water at 70–80 °C for the heating network. When the geothermal
water ﬂows through the evaporator, its heat is transferred to the working ﬂuid in the GHPS, and
its temperature is decreased. When the working ﬂuid ﬂows in the condenser, its heat is transferred
to the heating network water, keeping the temperature of the water at a high level. Here, we can
easily understand that working ﬂuids are very important to the GHPS. Currently, R22 is used
in most of the GHPS, most of which have a COP around 3 .
Despite its many good properties, there are many undesirable aspects about R22. Firstly, R22,
as a member of the HCFC family, will not be adopted for its high ODP (ozone depletion po-
tential, relative to R12) and GWP (greenhouse warming potential, relative to CO2 ). Secondly, it
has too high saturation pressure at a relatively high condensation temperature. For example, at a
condensation temperature around 70 °C, its saturation pressure is over 30 bar, exceeding the limit
of ordinary reciprocating compressors, say 25 bar. Thirdly, when the condensation temperature is
close to the critical temperature, the COP decreases rapidly. Because these reasons, the conden-
sation temperature of the GHPS using R22 can only be up to 60 °C. In other words, the heating
network water can only reach around 52 °C . The feed water temperature and return water
temperature are generally designed at 70–95 °C in a heating network system, much higher than the
GHPS outlet water temperature. Therefore, if the GHPS using R22 is used in a heating network
system, extra cost has to be incurred to overcome the lower than desired outlet water temperature.
A feasible alternative to the problem is a non-azeotropic working ﬂuid for the GHPS. It should be
compatible with environment conservation. Its saturation pressure should be lower than 25 bar
when the condensation temperature is at 80–100 °C, ensuring that the feed water temperature can
be at least 70 °C. Its heat capacities should be higher than 2.5 J/cm3 , ensuring that the working
ﬂuid does not overexpand and the boundary dimensions of the GHPS do not get too large.
Up to now, many substitutes for R22 have been proposed, such as R410A and R407C.
However, their saturation pressures are still too high when the condensation temperature is at 80–
100 °C. Usually used as working ﬂuids at high operating temperatures, R114 and R123 are seldom
used for GHPS with a reciprocating compressor due to their small heat capacities. They could be
used for GHPS with a centrifugal compressor, but such a compressor has an undesirably large
size. Some researchers have attempted to use mixtures of R12, R11 and R22, which again cause
environmental issues . To sum up, it is necessary to seek some working ﬂuids that can be used in
2. The working ﬂuids option
It may be challenging to ﬁnd suitable working ﬂuids among a lot of options. In this study, this
was done in the following procedure.
L. Zhao / Energy Conversion and Management 45 (2004) 1369–1378 1371
Firstly, the NIST REFPROP 4.0 (a piece of software for calculating thermophysical properties)
was used in order to seek some working ﬂuids that might be suitable to be used in GHPS. As a
result, seven mixture working ﬂuids were found, such as (R142b and R290), (R152a and R123),
(R124, R123 and R290), (R290, R600a and R123) etc.
Secondly, these seven mixture working ﬂuids were tested in a small experimental GHPS with a 2
kW compressor. Many experiments were performed with the condensation temperature up to 80–
100 °C. Data were collected by a computer when the system had reached a steady state. Through
comparing the experimental data for the seven mixtures, our search was narrowed to three, (R290,
R123 and R600a), (R290, R152a and R123) and (R290 and R123).
Thirdly, after analysis of the exergy losses of the three mixtures in the GHPS, the mixture
(R290, R600a and R123) was found to be the best and was, therefore, recommended for the
Lastly, the mixture was re-examined in consideration of environmental protection and other
issues. R290 and R600a, as hydrocarbon substances, are environment friendly with zero ODP and
GWP at the same level as CO2 . Other advantages of these two substances include low freezing
points, easy availability, non-corrosiveness for metals and dissolvability with lubricants. However,
they are combustible, so some ﬂame retardants should be mixed with them. R123, the third
substance in the recommended mixture, is a sort of nonﬂammable halohydrocarbon. Its ODP is
only 0.02 and GWP 29, and it can only exist in the atmosphere for 1.53 years, which means that
it will have little harmful eﬀect on the environment. Therefore, R123 can be used as the ﬂame
retardant for the mixture working ﬂuids.
Based on the above analyses, the authors proposed the non-azeotropic mixture working ﬂuid,
that is composed of R290 (propane, C3 H8 ), R600a (isobutene, C4 H10 ) and R123 (1,1 dichloro-
2,2,2 triﬂuroethane, C2 HCl2 F3 ) (50/10/40, wt.%). The properties of the mixture were experi-
mentally evaluated in our GHPS.
3. The geothermal heat pump system
The GHPS is installed for heating purposes in the Tianjin Geothermal Research and Training
Center in early winter and late spring in conjunction with the main heating network in the same
building. The following experiment was performed on April 6, 2001, and during the experiment,
the environment temperature changes by about 9–14 °C.
3.2. The heat consumer
The heat consumer is a building of 3500 m2 . On a 50 W/m2 basis, the buildingÕs total heating
load is about 175 kW. The feed water temperature and the return water temperature are designed
at 75 and 65 °C, and thus, the water ﬂow rate is determined at about 15 m3 /h. In the experiment,
the water ﬂow rate is 13.75 m3 /h because the environment temperature is higher than the design
temperature. When a balance has been reached between the GHPS export heat and the buildingÕs
1372 L. Zhao / Energy Conversion and Management 45 (2004) 1369–1378
heating load during the experiment, the water ﬂow rate in the condenser is reduced from 13.75 to
10.55 m3 /h to increase the temperature of the heating network water.
3.3. The geothermal water
The geothermal water is used as the low temperature source of the GHPS. Its design tem-
perature of 45 °C is decreased to 35 °C after the evaporator. After the evaporator, the water was
disposed into the city drainage system. Its ﬂow rate should be at about 11.5 m3 /h, but the ﬂow rate
remains at about 10.5 m3 /h because of the high environment temperature.
3.4. The GHPS
Two 22 kW reciprocating compressors are used in the GHPS. The condenser and the evapo-
rator are brazed plate heat exchangers. The condensation area is 11.21 m2 , and the evaporation
area is 9.31 m2 . The throttling device is a thermostatic expansion valve. In addition, there are a
sub cooler, a superheater, a dry ﬁlter, a ﬂuid storage tank etc. in the system. The GHPS is charged
with 80 kg of the non-azeotropic mixture working ﬂuid, which consists of 40 kg of R290, 32 kg of
R123 and 8 kg of R600a. The thermal sensorÕs precision is 0.1 °C and the pressure sensorÕs
precision is 0.001 bar. The geothermal and heating network water ﬂow rates are measured by two
ultrasonic ﬂow meters whose precision is 0.01 kg/s. The system and its components are shown in
Fig. 1. It is noted that although not shown in Fig. 1, a pump is used in each of the two cycles of
geothermal water and heating network water. The power consumption of the two pumps is not
considered in the total power consumption of the GHPS. As expected, the COP would be
somewhat lower otherwise.
3.5. The ﬂuid ﬂows
Fig. 2 schematically shows the ﬂuid ﬂows in the system. The working ﬂuid is compressed by two
parallel compressors (1, 13) and then enters the condenser (2), where its heat is transferred to the
water for the heating network, causing the working ﬂuid to change from vapor to liquid. Then the
working ﬂuid passes through the ﬂuid storage tank (3) and back pressure valve (4). At the exit of
the valve, the ﬂuid is divided into two streams. The smaller one ﬂows through expansion valve (5),
where it expands and is cooled and then enters the tubes inside the shell tube type sub cooler (6),
where it absorbs heat from the other stream on the other side of the tubes. Upon leaving the sub
cooler (6) and entering the gas-liquid separator (7), the ﬂuid in the smaller stream has changed
into gas. The larger stream, after being separated from the smaller one, goes through the sub
cooler (6) and dry ﬁlter (7), expands and gets cooled in expansion valve (8) and comes into the
evaporator (9), where the working ﬂuid evaporates by absorbing heat from the geothermal water
on the other side. After passing back pressure valve (10), the working ﬂuid enters the tubes of the
shell tube type superheater (11), where it is heated a second time by the geothermal water outside
the tubes. The superheated working ﬂuid leaves the superheater (11) and merges with the smaller
stream, and together they enter the gas-liquid separator (12). The working ﬂuid out of the gas-
liquid separator (12) again goes into compressors (1) and (13). This is a complete cycle of the
working ﬂuid ﬂow. The addition of the sub cooler and the superheater helps increase the COP of
L. Zhao / Energy Conversion and Management 45 (2004) 1369–1378 1373
Fig. 1. Schematic of the geothermal heat pump system.
Fig. 2. The ﬂuid ﬂows in the GHPS.
the GHPS. The condensation and evaporation pressures can be adjusted with the two back
pressure valves at the exits of the condenser and the evaporator to stabilize the system operation.
4. Experimental results and discussion
The experiment takes 5.5 h. Because the heating network water temperatures at the condenser
inlet and outlet are low at the beginning of the experiment, only data collected 3 h after the ex-
periment begins are taken as eﬀective. During the ﬁrst 3 h, the COP decreases from 4.5 to 3.7, the
heating network water at the condenser exit increases from 50 to 74 °C, the geothermal water
temperature at the evaporator inlet remains at 40–46 °C and the geothermal water temperature at
the evaporator exit keeps in the range of 31–36 °C. In other words, the geothermal water tem-
perature decreases by about 10 °C from the inlet to the exit. Therefore, the GHPS suﬃciently
1374 L. Zhao / Energy Conversion and Management 45 (2004) 1369–1378
utilizes the geothermal energy and reduces the discharge geothermal water temperature. The
experimental results are shown in Figs. 3–5. During the 3 h, the geothermal water temperature is
not able to be stabilized because of the temperature adjustment lagging. In fact, when the ex-
periment begins, the geothermal water is low at 40 °C, but the temperature rises slowly with
experiment continuing. At 2.5 h, the water temperature is above 45 °C. The temperature of the
cool water going into the geothermal water results in the geothermal water temperature being
reduced to about 41 °C. To improve the status, some controllers with accuracy will be used in the
future. Though the geothermal water temperature is changing, the heat network water temper-
140 160 180 200 220 240 260 280 300 320 340
Fig. 3. Variation of COP with time.
Inlet of condenser
72 Outlet of condenser
140 160 180 200 220 240 260 280 300 320 340
Fig. 4. Variation of the water temperature at the condenser inlet and outlet with time.
L. Zhao / Energy Conversion and Management 45 (2004) 1369–1378 1375
Fluids pressure at inlet of condenser(bar)
140 160 180 200 220 240 260 280 300 320 340
Fig. 5. Variation of the working ﬂuids pressure at condenser inlet with time.
ature reaches a balance, and since the COP is aﬀected mainly by the network water, the experi-
mental data are basically correct.
The variation of COP with time is shown in Fig. 3. In the early stage of the experiment, the
heating network water temperature at the condenser inlet is relatively low, so the condensation
temperature is low, too. On the other hand, the geothermal water temperatures at the evaporator
inlet and outlet only change in small ranges. These factors contribute to a relatively high COP in
this stage of the experiment. With the system approaching stabilization, the heating network water
temperature at the condenser inlet is rising, resulting in an increasing condensation temperature
and more compressor power consumption. As a result, the COP is lowered. When the system has
reached steady state, the COP remains at 3.9. In the late stage of the experiment, the COP de-
creases to 3.72 due to the artiﬁcial reduction in the ﬂow rate of the heating network water. The
water ﬂow rate is reduced because the temperature has reached 22 °C in the building. On the other
hand, more experimental data about diﬀerent GHPS operations are necessary to master the
characters of the GHPS.
Fig. 4 shows the variation of the heating network water temperature with time. In the beginning
of the experiment, the water temperature at the condenser inlet is 52.8 °C. As the experiment
proceeds, the temperature increases rapidly as the GHPS heating capacity is more than the
building heat load at this time. Later on, from the 180th min to the 300th min, as the heat balance
is being established, the temperature increases very slightly and ﬁnally stays at 63.0 °C. Aﬀected
by the inlet temperature, the condenser outlet temperature shows a similar trend, increasing from
around 60 °C in the beginning to 74 °C when stabilized but keeps about 10 °C higher than the inlet
temperature. The outlet temperature can be raised further, above 80 °C, to meet the requirements
of general residence heating design if the ﬂow rate of the heating network water is reduced.
Fig. 5 shows the variation of another important parameter, the working ﬂuid pressure at the
condenser inlet, with time. It can be assumed that it is equal to the pressure at the compressor
outlets, since the two locations are very close and, thus, the pressure loss is small. This pressure is
required to be lower than 25 bar for most reciprocating compressors. In our experiment, the
1376 L. Zhao / Energy Conversion and Management 45 (2004) 1369–1378
Some experimental data and calculated results
Measured parameters 150 min. 180 min. 210 min. 240 min. 270 min. 300 min. 330 min.
Working ﬂuid pressure at 5.61 5.61 5.60 5.61 5.61 5.96 5.96
evaporator inlet (bar)
Working ﬂuid pressure at 5.46 5.46 5.46 5.46 5.46 5.62 5.62
evaporator outlet (bar)
Working ﬂuid pressure at 13.80 15.50 16.52 17.15 17.15 17.15 17.70
condenser inlet (bar)
Working ﬂuid pressure at 13.42 15.08 16.13 16.74 16.74 16.75 17.34
condenser outlet (bar)
No. 1 compressor discharge 83.26 99.20 97.61 95.71 97.98 94.63 94.55
temperature (°C )
No. 1 compressor suck 41.62 41.68 41.67 42.15 40.36 40.32 40.30
temperature (°C )
No. 2 compressor discharge 75.21 91.96 89.53 90.14 91.43 89.65 89.25
temperature (°C )
No. 2 compressor suck 41.65 41.66 41.65 42.20 40.38 40.35 40.36
temperature (°C )
Working ﬂuid temperature at 70.91 88.09 85.49 85.66 86.70 84.81 85.28
condenser inlet (°C )
Working ﬂuid temperature at 62.43 71.31 75.89 77.57 79.23 79.44 79.15
condenser outlet (°C )
Working ﬂuid temperature at 33.87 33.91 32.10 31.57 30.98 30.48 30.85
evaporator inlet (°C )
Working ﬂuid temperature at 40.55 41.42 41.46 40.85 39.62 39.14 39.15
evaporator outlet (°C )
Water temperature at condenser 52.80 57.31 59.82 61.70 62.13 62.14 63.00
inlet (°C )
Water temperature at condenser 60.02 67.16 69.05 71.50 71.50 71.51 73.62
outlet (°C )
Geothermal water temperature at 43.80 44.99 44.79 45.38 42.74 41.27 41.16
evaporator inlet (°C )
Geothermal water temperature at 35.96 34.14 34.24 34.84 32.35 31.56 32.34
evaporator outlet (°C )
Condenser water ﬂow rate (m3 /h) 13.65 13.71 13.75 13.68 13.72 13.70 10.55
Geothermal water ﬂow rate (m3 /h) 10.46 10.38 10.66 10.34 10.13 10.86 10.27
Compressor input current (A) 48.60 70.81 70.58 76.24 73.09 72.80 66.94
Compressor input voltage (V) 375 376 376 376 378 378 377
The GHPS heating capacity (kW) 114.4 156.8 147.4 155.7 149.3 149.1 130.1
Heat released from geothermal 95.2 130.8 130.6 126.5 122.2 122.4 105.2
Actual compressor power 25.25 36.89 36.77 39.72 38.28 38.13 34.97
Actual COP of the GHPS 4.53 4.25 4.01 3.92 3.90 3.91 3.72
Note: during calculating in the table, the water density is 1000 kg/m3 .
L. Zhao / Energy Conversion and Management 45 (2004) 1369–1378 1377
pressure is at about 14 bar initially and at 18 bar at maximum. Therefore, the GHPS with is the
working ﬂuid can operate safely and stably in the long term. At the end of the experiment, the
working ﬂuids pressure at the condenser inlet rises clearly because the heating network water ﬂow
rate artiﬁcial reduction results in the working ﬂuids temperature rising at the condenser inlet.
Besides the variations of the four parameters shown in Figs. 3–5, other data are presented
in Table 1.
The compressor actual power consumption is calculated as
W ¼ 3 Á COSu Á U Á I=1000 kW ð1Þ
where U , I and COSu are voltage (V ), current (A) and power factor, respectively. The power
factor is taken as 0.8 in the calculation.
The GHPS heating capacity is calculated as
Q ¼ cp Á G Á q Á DT =3600 kW ð2Þ
where cp , G; q and DT are speciﬁc heat at constant pressure (4.186 kJ/kg °C), ﬂow rate (m3 /h),
water density (1000 kg/m3 ) and the temperature diﬀerence (°C) between the condenser inlet and
The heat released from the geothermal water in the evaporator is calculated in a similar way to
The actual COP of the GHPS is determined as:
COP ¼ Q=W ð3Þ
Based on the experimental investigation, some conclusions are drawn as follow.
The GHPS with the non-azeotropic mixture working ﬂuid R290/R600a/R123 (50%/10%/40%)
can operate under practical conditions. The condensation temperature is over 80 °C with the
condensation pressure below 18 bar and the compressor discharge temperature below 100 °C. The
COP of the GHPS is generally over 3.5. The water temperatures at the condenser or evaporator
inlet and outlet diﬀer only by 10 °C, indicating a large heat capacity per unit volume of the
mixture working ﬂuid. All these facts show that the non-azeotropic mixture working ﬂuid is
superior to R22 or R123.
The application of this GHPS technology can lower the discharge geothermal water temper-
ature and, thus, increase the eﬃciency of using geothermal energy and decrease the pollution to
the earth surface. Compared with conventional coal burning heating boilers, this is a clean,
eﬃcient and easy to maintain solution to heating needs.
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